Although useful in many commercial applications, polyolefins suffer a major deficiency, i.e., poor interaction with other materials. The inert nature of polyolefins significantly limits their end uses, particularly, those in which adhesion, dyeability, paintability, printability or compatibility with other functional polymers is paramount. The poor compatibility of polyolefins is further evidenced in their use as coatings where weak adhesion between polyolefin and metal surface has not allowed the use of this material for the protection of metal. Furthermore, attempts to blend polyolefins with other polymers have been unsuccessful for much the same reasons, i.e., the incompatibility of two polymers.
In general, polyolefins, such as polyethylene and polypropylene, have been among the most difficult materials to chemically modify. For example, in direct polymerization reactions using early transition metal catalysts, it normally is difficult to incorporate functional group-containing monomers into olefin polymers due to catalyst poisoning by functional groups. (See J. Boor, Jr., Ziegler-Natta Catalysts and Polymerizations; Academic Press: New York, 1979). Similarly, in the post-polymerization reactions, the inert nature and crystallinity of the olefin polymers usually makes it very difficult to chemically modify the polymers under mild reaction conditions. In many cases, the reactions involve serious side reactions, such as crosslinking and degradation. Accordingly, it is clear that there is a fundamental need to develop a new chemistry which can address these problems.
In previous patents (U.S. Pat. Nos. 4,734,472; 4,751,276; 4,812,529; 4,877,846), there has been a systematic investigation of borane-containing polyolefins. The chemistry disclosed in those patents involves a direct polymerization using organoborane-substituted monomers and alpha-olefin monomers in Ziegler-Natta processes. The homo- and copolymers formed in accordance with the processes of those patents contain borane groups and are useful intermediates for preparing a series of functionalized polyolefins. Many new functionalized polyolefins with various molecular architectures have been obtained based on the chemistry that is disclosed in those patents. Moreover, it has been demonstrated that the addition of polar groups to polyolefins can improve the adhesion of polyolefins to many substrates, such as metals and glass. (See, Chung et al, J. Thermoplastic Composite Materials 6, 18, 1993). The chemistry of borane containing polymers has also been extended to the preparation of polyolefin graft copolymers, which involves a free radical graft-from reaction (See U. S. Pat. Nos. 5,286,800 and 5,401,805), and it has been shown that in polymer blends, the compatibility of the polymers can be improved by adding a suitable polyolefin graft copolymer which reduces the domain sizes and increases the interaction between domains (See Chung et al, Macromolecules 26, 3467, 1993; Macromolecules, 27, 1313, 1994).
Diblock copolymers are an interesting class of materials, which exhibit some useful combinations of physical properties. The applications of diblock copolymers as compatibilizers are particularly useful in polymer blends (See U.S. Pat. No. 4,299,931; Cohen, et al, Macromolecules 15, 370, 1982; Macromolecules 12, 131, 1979; J. Polym. Sci., Polym Phys. 18, 2148, 1980; and U.S. Pat. No. 4,174,358). The incompatible polymers can be improved by adding a suitable compatibilizer which alters the morphology of these blends as well as interfacial adhesion between domains.
In chemistry, most diblock copolymers have been produced by sequential living polymerization processes, namely anionic (See U. S. Pat. No. 3,265,765), cationic (See U.S. Pat. No. 4,946,899) and metathesis (See R. H. Grubbs, et al, Macromolecules 21, 1961, 1988) living polymerizations. However, living polymerization processes are limited to a relatively small number of monomers which undergo living propagation, and are limited further by difficulties in the crossover reaction from one monomer to the other. The extension of sequential living polymerization to transition metal coordination polymerization for preparing polyolefin diblock copolymers has been very limited. Only a few cases have been reported under very inconvenient reaction conditions and with special catalysts (See Y. Doi, et al, Makromol. Chem. 186, 11, 1985; Adv. Polym Sci., 73/74, 201, 1989 and H. Yasuda, et al, Macromolecules, 25, 5115, 1992). Other methods of prepareing polyolefin diblock copolymers involve a transformation reaction from anionic to Ziegler-Natta polymerization, (See R. E Cohen, J. Polym. Sci.: Part A: Polym. Chem. 24, 2457, 1986) or from Ziegler-Natta to free radical vinyl polymerization (See U.S. Pat. No. 3,887,650), and a coupling reaction (See R. Mulhaupt et al, Makromol. Chem., Macromol. Symp. 48/49, 317, 1991). In general, the product of such a coupling reaction most likely is an intimate mixture of homopolymers and perhaps some block copolymer; and based on measured lifetimes of the growing chains and efficiency of the coupling reaction, the yields of polyolefin diblock copolymers are well below 20%.
Recently, a method of preparing polypropylene diblock copolymers from vinylidene-terminated polypropylene has been reported (See Chung et al, Macromolecules 32, 2525, 1999; Macromolecules 31, 5943, 1998; Polymer 38, 1495, 1997; and Mulhaupt et al, Polymers for Advanced Technologies, 4, 439, 1993). The vinylidene-terminated polypropylene, also referred to as chain end unsaturated polypropylene, can be prepared by metallocene polymerization or thermal degradation of high molecular weight polypropylene (PP). The overall diblock copolymer chemistry involves a multiple-step chain extension process which is started with a functionalization reaction of the vinylidene group at the PP chain end. The formed terminal functional group is then used as the active site for coupling reactions or chain extension reactions, such as free radical and ring-opening polymerizations, to produce PP diblock copolymers. Overall, the effectiveness of this chain extension process is strongly dependent on (a) the percentage of polymer chains having a vinylidene terminal group and (b) the efficiency of functionalization reaction. It is obvious that the efficiency of functionalization reaction reduces with an increase of PP molecular weight, due to the reduced vinylidene concentration. Some functionalization reactions are very effective for low molecular weight PP. However, they become very difficult for PP polymers having a molecular weight higher than about 30,000 g/mole. Unfortunately, for many applications, the use of high molecular weight PP is essential. For example, for improving the interfacial interactions in PP blends and composites, it is essential that the PP have a high molecular weight. In addition, the availability of chain-end unsaturated polyolefin is very limited. Most polyolefins, except polypropylene, have only a low percentage of chain-end unsaturation in their polymer chains.
In general, developments in metallocene homogeneous catalysis have provided a new era in polyolefin synthesis (See, for example, U.S. Pat. Nos. 4,542,199, 4,530,914, 4,665,047, 4,752,597, 5,026,798 and 5,272,236). With well-designed (single-site) catalyst systems, monomer insertion can be greatly enhanced. This is especially important for copolymerization reactions involving high alpha-olefins and styrenic monomers. In this latter regard, it has been disclosed to use metallocene catalysts having a constrained ligand geometry for producing linear low density polyethylene (LLDPE), poly(ethylene-co-styrene) (U.S. Pat. No. 5,703,187), poly(ethylene-co-p-methylstyrene ), poly(ethylene-ter-propylene-ter-p-methylstyrene) and poly(ethylene-ter-1-octene-ter-p-methylstyrene) (U.S. Pat. Nos. 5,543,484 and 5,866,659), with narrow molecular weight and composition distributions. The relatively opened active site in a catalyst with constrained ligand geometry facilitates the incorporation of relatively large amounts of high olefinic monomers in olefin copolymers. In general, the incorporation of high alpha-olefins is significantly higher when using opened active site catalysts having constrained ligand geometry than when using traditional Ziegler-Natta catalysts. In several publications, the experimental results also show that effective chain transfer reaction can take place with hydrogen and silane in some metallocene polymerization reactions. Several organosilanes having Si-H groups are effective chain transfer agents in metallocene-mediated polymerizations that result in silane-terminated polyolefins and copolymers (See Marks, J. Am. Chem. Soc. 1998, 120, 4019; J. Am. Chem. Soc. 1995, 117, 10747; Macromolecules 1999, 32, 981).